Allowance for credit losses - Allowance Rollforward (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance, beginning of year		$ 500
Provision for (reduction of ) credit losses		85	$ (34)	$ (96)
Balance, end of year		500	500
Conventional Mortgage Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance, beginning of year		500	500	650
(Charge-offs) recoveries		(85)	34	(54)
Provision for (reduction of ) credit losses		85	(34)	(96)
Balance, end of year		500	500	500
Ending balance, individually evaluated for impairment		0	0	0
Ending balance, collectively evaluated for impairment		500	500	500
Recorded Investment Individually evaluated for impairment	[1]	13,395	15,402	17,668
Recorded Investment Collectively evaluated for impairment	[1]	$ 4,212,224	$ 3,969,142	$ 3,632,833

[1] These amounts exclude government mortgage loans because we make no allowance for credit losses based on our investments in government mortgage loans, as discussed above under — Government Mortgage Loans Held for Portfolio.